sumatra_earmark lets you earmark any pdf page in SumatraPDF
Each earmarked pdf page displays a blue square in the top right corner
Quickly toggle earmarks on/off with mouse or keyboard
Quicky jump to next/prev earmark page
Earmarks are autosaved to a textfile
Any pdf with at least one earmark shows a grey square in top right corner
Handy popup grid for quick jump to a specific earmark
Unique earmarks for different pdf files (based on file name)
The pdf files are not edited

Supports 255 earmarks per unique pdf file name
Autosaves earmarks in [Script Folder]\sumatra_earmarks.txt
Also works on djvu files in SumatraPDF
The blue earmark lags behind a bit if you move the pdf window quickly
Inspired by paper book earmarks (duh!) and line bookmarks in Notepad++

Setup

Download, unzip and run sumatra_earmarks.exe
Place a link in the Startup folder to autorun it Windows starts.

Notes

sumatra_earmarks is really "feature request ware". That is, I hope the SumatraPDF devs like this and build in the functionality.
Made in Autohotkey by nod5 as Free Software GPL3
Tested in Win7 x64. The zip contains binary and source.